drm/ofdrm: Support big-endian scanout buffers

All DRM formats assume little-endian byte order. On big-endian systems,
it is likely that the scanout buffer is in big endian as well. Update
the format accordingly and add endianness conversion to the format-helper
library. Also opt-in to allocated buffers in host format by default.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221011150712.3928-6-tzimmermann@suse.de